| Deformation of bulk polycrystalline materials often results in the development of dislocation slip bands and deformation twins in individual grains. Evolution of deformation microstructures is closely tied to the plasticity and service conditions of the material. This study employed two modern characterization tools, Electron Backscattered Diffraction (EBSD) and Differential Aperture X-ray Microscopy (DAXM), to study deformation twins and dislocations developed in deformed polycrystalline Ti. Through extensive EBSD characterization, it was found that many {101¯2}<1¯011> T1 twins nucleated from grain boundaries through a slip transfer process: dislocations gliding on a prismatic slip system stimulated twin nucleation in a neighboring grain. This slip transfer twin nucleation mechanism requires a good geometric alignment between the active slip system in one grain and the stimulated twinning system in the neighboring grain, represented by a high slip transfer parameter m'. The value of m' dictates which twinning system will be activated and suggests at which grain boundaries slip-stimulated twin nucleation will occur. Another important scenario for twin nucleation features pairs of twins that are connected with each other at grain boundaries. Statistical analysis of existing examples of paired twins indicates that the two operating twinning systems often have a good geometric alignment (high m'), and at least one of them must have a high Schmid factor. Paired twins are more often observed at low angle grain boundaries than at high angle grain boundaries. This finding was interpreted using the m' rule. DAXM is a non-destructive technique for characterizing subsurface microstructure, which reveals grain size, grain boundary inclination, and dislocation content. A method that relates the direction of peak streak in Laue patterns with local geometrically necessary dislocations (GNDs) was established. This method was used to analyze residual GNDs near grain boundaries. Combining high-resolution EBSD and DAXM, a comprehensive study of {112¯1}<1¯1¯26> T2 twins was conducted. Most T2 twins nucleated at grain boundaries with one or more tensile twins formed from the same place in the neighboring grain. Significant orientation gradients were observed in some T2 twins. Small T1 twins, transition regions with intermediate orientations, surface ledges, and microcracks were often observed near T2 twin boundaries. <c+a> dislocations, whose Burgers vector belong to the T2 twinning plane, were also identified in the vicinity of some T2 twins. Overall, this work provides new knowledge of microscopic deformation mechanisms in Ti, especially with regard to twinning, which can be incorporated into crystal plasticity constitutive models. In addition, the research methodology developed in this work is beneficial for other metallurgical studies. |
